GB2086676A

GB2086676A - Protective circuit for electric bedcover or the like

Info

Publication number: GB2086676A
Application number: GB8129993A
Authority: GB
Original assignee: Sunbeam Corp
Current assignee: Sunbeam Corp
Priority date: 1980-11-03
Filing date: 1981-10-05
Publication date: 1982-05-12
Also published as: GB2086676B; FR2493625A1; FR2493625B1; JPS57105989A; AU541235B2; AU7701481A; MX149865A; NZ197765A; DE3137754A1; CA1162598A

Abstract

A protective circuit for an electric bedcover or similar electric appliance comprises a sensing wire 22, which is insulated electrically from a heating wire 18 by an insulative material 20 having a fusing temperature corresponding to an overheated condition, a thermal switch 60, which disables the heating wire when heated sufficiently, an auxiliary heater 88, which is coupled thermally to the thermal switch 60 and is connected to the sensing wire, a first diode 110 which is connected between a first end 18a of the heating wire 18 and the auxiliary heater 88, a second diode 120, which is connected between a second end 186 of the heating wire 18 and the auxiliary heater 88, and a capacitor 130, which is connected in parallel with the heating wire. The thermal switch may be a thermistor which reestablishes supply to the heating wire 18 on cooling. Alternatively, an electromagnetic relay may be substituted for the thermal switch and the auxiliary heater. <IMAGE>

Description

SPECIFICATION Protective circuit for electric bedcover or the like This invention pertains to a protective circuit for an electric bedcover, electric heating pad, or other electrical appliance comprising a supporting substrate, which may be a fabric shell of an electric bedcover or similar electric appliance, and a heating wire, which traverses the supporting substrate. Such an appliance comprising a fabric shell may be a heated muff, boot, garment, pillow, etc.

Prior protective circuits of pertinent interest are disclosed in U.K. Patent Application No.

2,028,608A. In each of such protective circuits, a heating wire is arranged to be supplied with an alternating current, and a sensing wire is insulated electrically from the heating wire by temperature-sensitive material, which in some disclosed embodiments melts to effect electrical contact between the sensing wire and the heating wire in an overheated condition. Similar uses of temperature-sensitive materials were exemplified in U.S. Patent No. 3,493,727, U.S. Patent No. 3,628,093, British Patent Specification No. 1,155,118, and British Patent Specification No.

1,456,684.

In each of the protective circuits disclosed in U.K. Patent Application No. 2,028,608A, a first diode and a first resistor in series are connected between a first end of the heating wire and the sensing wire, a second diode and a second resistor in series are connected between a second end of the heating wire and the sensing wire, and polarities of the two diodes are such as not to allow current to bypass the heating wire through the two diodes. The resistors are coupled thermally to a thermal fuse, which is connected so as to disable the heating wire when heated sufficiently.

In the protective circuits disclosed in U.K.

Patent Application No. 2,028,608A, an overheated condition at either extreme end of the heating wire causes the insulative layer between the sensing wire and the heating wire to melt so as to allow current to flow at alternate half-cycles of the alternating current supplied to the heating wire through the sensing wire, and through the particular resistor and the associated diode connected to the opposite end of the heating wire, whereupon the conducting resistor heats the thermal fuse.

The other resistor does not conduct as there is no drop in voltage across it. An overheated condition at an intermediate part of the heating wire thus allows current to flow alternatingly through the respective resistors. U.K. Patent Application No. 2,028,608A also discloses that the diodes are protected against reverse transient damage by the resistors.

Other protective circuits of pertinent interest are disclosed in U.K. Patent Application No.

2,028,607A. In each of such protective circuits, a sensing wire is insulated electrically from a first heating wire by temperature-sensitive material, which may melt in like manner, and a second heating wire is interlocated with the first heating wire and connected at a given end to the first heating wire so as to divide an applied voltage between the heating wires, which are arranged to be supplied with an alternating current. Also, both ends of the sensing wire are connected to the opposite end of the second heating wire through a resistor, which is coupled thermally to a thermal fuse. In one such circuit, as shown in Figs. 3 of U.K. Patent Application No.

2,028,607A, a diode is connected between the respective heating wires.

As discussed in U.K. Patent Application No.

2,028,607A and U.K. Patent Application No.

2,028,608A, some types of temperature-sensitive material have negative temperature coefficients of resistance. Pertinent information of related interest may be found in U.S. Patent No. 2,581,212, U.S. Patent No. 2,846,559, and U.S. Patent No. 2,846,560.

Other protective circuits of pertinent interest are disclosed in a co-pending patent application filed on May 30, 1980, by John C.

Rentz, under U.S. Serial No. 1 55,033, and assigned commonly herewith.

Additional background may be obtained from U.S. Patent No. 2,195,958, U.S. Patent No. 4,205,223, British Patent Specification No. 1,566,005, French Patent Publication No. 2,406,330, French Patent Publication No. 2,408,932, and French Patent Publication No. 2,416,611.

This invention provides an improvement in a protective circuit for an electric appliance comprising a supporting substrate and a heating wire, which traverses the supporting structure, and which is adapted to be supplied with an alternating current. As mentioned above, the supporting substrate may be a fabric shell, within which the heating wire is deployed.

The protective circuit improved by this invention comprises a sensing wire, which is deployed with the heating wire, which is insulated electrically from the heating wire by an insulative layer being deployed along the heating wire and having a fusing temperature corresponding to an overheated condition of the electric appliance, and which is interwound with the heating wire so as to effect electrical contact between the sensing wire and the heating wire if at least part of the insulative layer fuses between the sensing wire and the heating wire.

The protective circuit improved by this invention comprises a thermal means, which disables the heating wire when heated sufficiently, and an auxiliary heater, which is coupled thermally to the thermal means, and which is connected to the sensing wire at a first end of the auxiliary heater. Preferably, the thermal means comprises a fusible junction, which opens when heated sufficiently.

Alternatively, the thermal means may comprise a temperature-triggered switching device having a positive temperature coefficient of resistance.

As improved by this invention, the protect tive circuit comprises a first diode, which is connected between a first end of the heating wire and a second end of the auxiliary heater, a second diode, which is connected between a second end of the heating wire and a second end of the auxiliary heater and so as not to allow current to bypass the heating wire through the first and second diodes, and a capacitor, which is connected in parallel with the heating wire, whereby an overheated condition causing at least part of the insulative layer to fuse closes an electrical circuit through the auxiliary heater, which thus is enabled to heat the thermal means sufficiently for the thermal means to disable the heating wire, and whereby the capacitor protects the first and second diodes against damage from reverse spikes of voltage.

Alternatively, an electromagnetically relay, which comprises normally closed contacts and a coil coupled magnetically to the contacts, and which is connected so as to disable the heating wire when the coil conducts sufficient current for the contacts to be opened, may be substituted for the thermal means and the auxiliary heater, whereby an overheated condition causing at least part of the insulative layer to fuse closes an electrical circuit through the coil, which conducts sufficient current for the contacts to be opened so as to disable the heating wire.

Furthermore, the insulative layer may be made of a selected material having a negative temperature coefficient of resistance, whereby an overheated condition of less severity causes the insulative layer to conduct leakage current through the auxiliary heater, or through the coil if used, before the insulative layer fuses.

In an overheated condition at the first end of the heating wire, the auxiliary heater and the second diode conduct alternate half-cycles (of a given polarity) of the alternating current supplied to the heating wire. In an overheated condition at the second end of the heating wire, the auxiliary heater and the first diode conduct alternate half-cycles (of the opposite polarity) of such alternating current. In an overheated condition at an intermediate part of the heating wire, the auxiliary heater and the second diode conduct alternate half-cycles (of the given polarity) of such alternating current, and the auxiliary heater and the first diode conduct alternate half-cycles (of the opposite polarity) of such alternating current.

Advantageously. as reverse spikes of wolt- age exhibit higher frequencies than line frequency, and as capacitive reactance of the capacitor varies inversely with the frequency, the capacitor conducts reverse spikes of current preferentially so as to protect the diodes against damage.

These and other objects, features, and advantages of this invention will be evident from the following description of a preferred embodiment of this invention.

Figure 1 is a fragmentary, partly brokenaway, elevational view of a coaxial cable comprising a heating wire, a sensing wire, and associated insulative layers, as used in an electric bedcover, electric heating pad, or other electric appliance embodying this invention.

Figure 2 is a cross-sectional view taken along line 2-2 of Fig. 1, in the direction of the arrows, and on an enlarged scale. Fig. 2 also shows a fabric shell, within which the coaxial cable is deployed.

Figure 3 is a circuit diagram of a protective circuit utilizing the coaxial cable of Figs. 1 and 2 and according to a preferred embodiment of this invention.

Figure 4 is a circuit diagram of a protective circuit utilizing the coaxial cable of Figs. 1 and 2 and according to an alternative embodiment of this invention.

Figure 5 is a pictorial view of a thermal switch, which comprises a fusible junction, and which is utilized in the protective circuit of Figs. 3 and 4.

Figures 6 and 7 are respective circuit diagrams of other protective circuits utilizing the coaxial cable of Figs. 1 and 2 and according to other alternative embodiments of this invention.

A protective circuit according to this invention utilizes a coaxial cable 10, which is shown in Figs. 1 and 2, and which is similar to prior coaxial cables, as exemplified in U.S.

Patent No. 3,628,093 and U.S. Patent No.

3,493,727. As shown in Fig. 2, the coaxial cable 10 is deployed in a network of serpentine channels 12, one of which is shown in cross-section, within a fabric shell 14, which is shown fragmentarily, of an electric bedcover, which may be an electric overblanket or an electric underblanket.

As shown in Fig. 1 and 2, the coaxial cable 10 comprises a textile core 16, which comprises numerous twisted strands, a first conductor 18, which is wound tautly around the textile core 16, an insulative layer 20, which surrounds the first conductor 1 8 and insulates the first conductor 1 8 electrically, and which fuses at a predetermined temperature corresponding to an overheated condition of the electric bedcover, a second conductor 22, which is wound tautly around the insulative layer 20 so as to be drawn into electrical contact with the first conductor 1 8 if at least part of the insulative layer 20 fuses between the second conductor 22 and the first conduc tor 18, and an insulative jacket 24, which surrounds the second conductor 22 and insulates the second conductor 22 electrically.

As shown in Fig. 1, the first conductor 1 8 and the second conductor 22 preferably are flat wires of rectangular cross-section and preferably are wound in opposite-hand shell ices, the first conductor 1 8 being wound in a right-hand helix as shown. As shown in Fig.

1, the helical pitch of the second conductor 22 thus is longer than the helical pitch of the first conductor 1 8. Alternatively, the first conductor 1 8 and the second conductor 22 may be round wires of circular cross-section, rather than flat wires of rectangular cross-section.

Similarly, the first conductor 1 8 and the second conductor 22 may be wound in samehand helices, as disclosed in U.S. Patent No.

3,493,727.

For the preferred embodiment of Fig. 3, the second conductor 22, which serves as a sensing wire, preferably is made of conventional signalling wire having a low resistance per unit length, whereupon the first conductor 1 8 is made of conventional heating wire having a high resistance per unit length so as to dissipate power at a given rate. For the alternative embodiment of Fig. 4, the second conductor 22, which serves as a sensing wire in one mode of operation and as a heating wire in another mode of operation, preferably is made of similar heating wire having a different resistance permit length so as to dissipate power at a different rate. Such signalling wire and such heating wire have other known uses in prior electric bedcovers, electric heating pads, and other electric applicances. A heating wire can serve as a sensing wire.

The insulative layer 20 may be made of polyethylene, or equivalent material, of any suitable grade having suitable electrical and mechanical characteristics including a fusing temperature corresponding to an overheated condition of the electric bedcover. In an electric bedcover, low-density polyethylene having a fusing temperature of approximately from 230"F to 250'F may be used for the insulative layer 20, and the textile core 1 6 may be made of numerous twisted strands of rayon (cellulosic) fiber or equivalent material. Because of the temperature gradient decreases radially outwardly in the coaxial cable 10, the insulative jacket 24 and the insulative layer 20 may be made of the same material.Alternatively, the insulative jacket 24 may be made of another suitable material having a fusing temperature or glass-transition temperature that is substantially higher than the fusing temperature of the insulative layer 20. In an electric bedcover, polyvinyl chloride having a glass-transition temperature of approximately from 300"F to 330"F may be used for the insulative jacket 24.

A thermal switch 60, which is utilized in the preferred embodiment of Fig. 3 and also in the alternative embodiment of Fig. 4, Is shown diagrammatically in Fig. 3 and pictorially in Fig. 5. The thermal switch 60, which is similar to prior thermal switches having other known uses in prior electric blankets, comprises a circuit board 62, which is made of conventional rigid, insulative material, a bronze base 64, which has a flat top 66, opposite downturned sides 68, and opposite lateral flanges 70, and which is mounted to the circuit board 62 by conventional rivets 72 engaging the lateral flanges 70.The thermal switch 60 also comprises a bronze strip 74, which has a mounting flange 76, a contacting flange 78, and a curved portion 80 between the mounting flange 76 and the contacting flange 78, and which is mounted to the circuit board 62 by another rivet 82 engaging the mounting flange 76. The bronze base 64 and the bronze strip 74 are good conductors of heat and current. The bronze strip 74, which acts as a spring leaf, has suitable spring characteristics.

The contacting flange 78 is soldered to the flat top 66 by a eutectic alloy providing a fusible junction 84, which opens when heated sufficiently, and which cannot be reset under normal operating conditions. The eutectic alloy may be composed of 63 parts of tin and 37 parts of lead, so as to fuse at approximately 361"F. Such a solder is available commercially from Newark Electronics Div., Premier Industrial Corp., 500 North Pulaski Road, Chicago, Illinois 60625. The bronze strip 74 is biased so as to separate from the bronze base 64 if the fusible junction 84 fuses.

Suitable thermal switches, which comprise suitable fusible junctions having specified fusing temperatures as low as 140"F, are available commercially from Emerson Electric Co., Micro Devices Division, 1 881 Southtown Boulevard, Dayton, Ohio 45439, under its trademark "Microtemp".

An auxiliary heater 88, which may be a conventional carbon or wire-wound resistor having opposite leads 88a, 88b, is mounted to the circuit board 62, by conventional grommets 96 receiving the leads 94, so as to be disposed beneath the flat top 66 for good transfer of heat from the auxiliary heater 88, through the bronze base 64, to the fusible junction 84. The thermal switch 60 and the auxiliary heater 88 may be encased in a protective capsule (not shown) outside the fabric shell 14, of a type used conventionally to encase controls for an electric bedcover, electric heating pad, or other electric appliance.

As shown in Fig. 3, the heating wire 1 8 preferably is energized by unrectified alternating current from a conventional source 100 through a switch 122, which may be an onoff switch, an ambient-responsive control, or any other conventional switch, as may be appropriate in an electric bedcover, electric heating pad, or other electric appliance. If the switch 1 22 is to be an ambient-responsive control in an electric bedcover, the switch 1 22 may be similar to the ambient-responsive control described in U.S. Patent No.

2,195,958.

In the preferred embodiment of Fig. 3, the first conductor 1 8 serves as a heating wire, and the second conductor 22 serves as a sensing wire. Both ends of the second conductor 22 are connected to each other at a shunting connection 102 and through the shunting connection 102 to a given lead 88a of the auxiliary heater 88. The opposite lead 88b of the auxiliary heater 88 is connected to a common connection 104: The shunting connection 102 provides two parallel paths for current through the auxiliary heater 88. If the second conductor 22 breaks so as to open one such path, the shunting connection 102 provides an alternate path for current through the auxiliary heater A first diode 110 is connected between a given end 1 8a of the first conductor 1 8 and the auxiliary heater 88.A second diode 120 is connected between the opposite end 1 8b of the first conductor 1 8 and the auxiliary heater 88. The first diode 110 and the second diode 1 20 are connected to the auxiliary heater 88 through the common connection 1 04. The respective polarities of the first diode 110 and the second diode 1 20 are such as not to allow current to bypass the first conductor 1 8 through the first diode 110 and the second diode 1 20. If the illustrated polarity of the first diode 110 is reversed, the illustrated polarity of the second diode 1 20 must be reversed, and vice-versa.

An overheated condition causing at least part of the insulative layer 20 to fuse closes an electrical circuit through the auxiliary heater 88, which thus is enabled to heat the thermal switch 60 sufficiently for the thermal switch 60 to open so as to disable the first conductor 1 8 serving as a heating wire.

In an overheated condition at the first end 1 8a of the first conductor 18, the auxiliary heater 88 and the second diode 1 20 conduct alternate half-cycles (of a given polarity) of the alternating current supplied to the first conductor 1 8 by the source 1 00. In an overheated condition at the second end 1 8b of the first conductor 18, the auxiliary heater 88 and the first diode 110 conduct alternate halfcycles (of the opposite polarity) of such alternating current.In an overheated condition at an intermediate part of the first conductor 1 8, the auxiliary heater 88 and the second diode 1 20 conduct alternate half-cycles (of the given polarity) of such alternating current, and the auxiliary heater 88 and -he first diode 110 conduct alternate half-cycles (of the opposite polarity) of such alternating current.

A capacitor 1 30 is connected in parallel with the first conductor 1 8. The capacitor 1 30 protects the first diode 110 and the second diode 1 20 against damage from reverse spikes of voltage.

For any capacitor, capacitive reactance in ohms is equal to 1 0.159 xc = ~~~~ - 2rrfC fC where f equals frequency in hertz and C equals capacitance in farads. Hence, as reverse spikes of voltage exhibit higher frequencies than line frequency, and as capacitive reactance of the capacitor 1 30 varies inversely with the frequency, the capacitor 1 30 conducts reverse spikes of current preferentially so as to protect the first diode 110 and the second diode 1 20 from damage.

In the alternative embodiment of Fig. 5, a multipole double-throw switch 140 enables alternative modes by operation to be selected.

In a first mode of operation, the first conductor 1 8 serves as a heating wire, which dissipates power at a given rate, and the second conductor 22 serves as a sensing wire, as in the preferred embodiment. In a second mode of operation, the second conductor 22 serves as a heating wire, which dissipates power at a different rate, and the first conductor 1 8 serves as a sensing wire.

The switch 140 has a pole 140a, which can be switched between a contact 1 40b for the first mode of operation and a contact 1 40c for the second mode of operation, and which is connected to the first diode 110, to a given side 130a of the capacitor 130, and to a given side 1 0Oa of the source 1 00. The contact 140b is connected to the given side 1 8a of the first conductor 1 8. The contact 1 40c is connected to a given side 22a of the second conductor 22.

The switch 140 has a pole 140d, which can be switched between a contact 140e for the first mode of operation and a contact 140f for the second mode of operation, and which is connected to the lead 88a of the auxiliary heater 88 at the shunting connection 1 02.

The contact 140e is connected to the given end 22a of the second conductor 22. The contact 140fis connected to the given end 1 8a of the first conductor 1 8.

The switch 140 has a pole 140g, which can be switched between a contact 140h for the first mode of operation and a contact 140for the second mode of operation, and which is connected to the lead 88a of the auxiliary heater 88 at the shunting connection 1 20. The contact 140h is connected to the opposite end 22b of the second conductor 22. The contact 140k is connected to the opposite end 1 8b of the first conductor 18.

The switch 140 has a pole 140p, which can be switched between a contact 140q for the first mode of operation and a contact 1 40r for the second mode of operation, and which is connected to the second diode 120, to the opposite side 1 30b of the capacitor 130, and through the thermal switch 60 and the switch 102 to the opposite side 1 00b of the source 100. The contact 1 40q is connected to the opposite side 1 8b of the first conductor 1 8. The contact 1 40r is connected to the opposite side 22b of the conductor 22.

Thus, the switch 1 40 enables the first conductor 1 8 and the second conductor 22 to be interchanged selectively, whereupon different rates of dissipation of power can be selected.

However, in other respects, the alternative embodiment of Fig. 4 is similar to the preferred embodiment of Fig. 3.

In either described embodiment, the source 100 may deliver unrectified alternating current at 110-120 VAC, 60 Hz, whereupon the carbon or wire-wound resistor constituting the auxiliary heater 88 must be of such size and value to raise the temperature of the fuse alloy to its melting point (361"F) in a reasonable time after an overheated condition effects electrical contact between a sensing wire and a heating wire. A resistor of 1 or 2 watt size with a resistance value between 500 ohms and 3,500 ohms suffices. The capacitor 1 30 may have a capacitance of about .01 microfarard.

Similarly, at 220-240 VAC, 50 Hz, a resistor of 1 or 2 watt size with a resistance value between 1,800 ohms and 8,200 ohms suffices. The exact values of resistance used depend on the design of the thermal switch and more specifically on its adequacy with respect to thermal conductivity between the resistors and the fusing alloy. The capacitor 1 30 may have a capacitance of about 0.005 microfarad.

As shown in Fig. 6, a thermistor 1 60 having a positive temperature coefficient of resistance may be used as a thermal switch and may be substituted for the thermal switch 60, in the embodiment of Fig. 3, so long as the thermistor 1 60 has suitable characteristics. The thermistor 1 60 is connected so as to disable the heating wire 1 8 when heated sufficiently, and so as to enable the heating wire 1 8 otherwise, as when the thermistor 1 60 is cooled sufficiently after the thermistor 1 60 has disabled the heating wire 18. Similarly, the thermistor 1 60 may be substituted for the thermal switch 60, in the embodiment of Fig. 4.

Thermistors having suitable characteristics are available commercially from Raychem Corporation, 300 Constitution Drive, Menlo Park, California 94025, as PolySwitch TM Thermal Limit Devices, and are described in Specification TLD-01, which is published by Raychem Corporation, and which describes such devices as temperature-triggered switching devices used for thermal protection of electrical equipment. Herein, all references to thermal means contemplate such devices and equivalent devices, as well as thermal switches having fusible links.

As shown in Fig. 7, an electromagnetic relay, which comprises normally closed contacts 1 70 and a coil 1 80 coupled magnetically to the contacts 170, and which is connected so as to disable the heating wire 1 8 when the coil 1 80 conducts sufficient current for the contacts 1 70 to be opened, may be substituted for the thermal switch 60 and the auxiliary heater 88, in the embodiment of Fig.

3, whereby an overheated condition causing at least part of the insulative layer 20 to fuse closes an electrical circuit through the coil 180, which conducts sufficient current for the contacts 1 70 to be opened so as to disable the heating wire 18. Similarly, such a relay may be substituted for the thermal switch 60 and the auxiliary heater 88, in the embodiment of Fig. 4.

In an embodiment described above, the insulative layer 20 may be made of a selected material not only having a fusing temperature corresponding to an overheated condition but also having a negative temperature coefficient of resistance, whereby an overheated condition of less severity causes the insulative layer 20 to conduct leakage current before the insulative layer 20 fuses.

In such an embodiment comprising the thermal switch 60 and the auxiliary heater 88, such current may be sufficient for the thermal switch 60 to open before the insulative layer 20 fuses, so as to disable the heating wire(s) without permanent damage to the insulative layer 20. Once opened, the thermal switch 60 cannot be reset automatically.

In such an embodiment comprising the thermistor 1 60 and the auxiliary heater 88, such current may be sufficient for the thermistor 1 60 to disable the heating wire(s) before the insulative layer 20 fuses, so as to provide modulated control of the heating wire(s) without permanent damage to the insulative layer 20. As compared to the thermal switch 60, the thermistor 1 60 becomes reset automatically if, as, and when the overheated condition of less severity passes.

In such an embodiment comprising the electromagnetic relay, which comprises the contacts 1 70 and the coil 1 80, such current may be sufficient for the contacts 1 70 to be opened, so as to provide modulated control of the heating wire(s) without permanent damage to the insulative layer 20. As well, such a relay becomes reset automatically if, as, and when the overheated condition passes.

In an electric bedcover embodying a protective circuit according to any embodiment described above, a suitable material having a negative temperature coefficient of resistance is doped polyvinyl chloride, which is available commercially from The B.F. Goodrich Co., B.F. Goodrich Chemical Div., Independence, Ohio, as "GEON" No. 82726-natural-024.

0.8% doped with "Triton X-400" dopant.

Claims

1. A protective circuit for an electric appliance comprising a supporting substrate traversed by a heating wire having a first end and a second end, and which is adapted to be supplied with an alternating current, the protective circuit comprising a sensing wire interwound with the heating wire and insulated electrically therefrom by an insulating layer of material having a relatively high impedance at normal working temperatures which falls to a relatively low value on overheating and/or melts to permit direct contact of the heating and sensing wires, a conductive element having a first and a second end the first end being connected to the sensing wire, a first diode connected between the first end of the heater wire and the second end of the conductive element and a second diode connected between the second end of the heating wire and the second end of the conductive element so as not to allow current to bypass the heating wire through the first and second diodes a capacitor connected in parallel with said heating wire to protect the first and second diodes against damage from reverse spikes of voltage, and means responsive to current flowing through said conductive element in the event of an overheated condition causing at least part of said insulating layer to fuse or reduce in impedance as aforesaid said current responsive means being adapted to disable the heating wire.

2. A circuit according to claim 1 wherein said conductive element comprises a resistor and said current responsive means comprises a thermal fuse in thermal contact with said resistor.

3. A circuit according to claim 2 wherein said thermal fuse comprises a fusible junction adapted to open when heated sufficiently.

4. A circuit according to claim 2 wherein said thermal fuse comprises a temperature triggered switching device having a positive temperature characteristic of resistance.

5. A circuit according to claim 1 wherein said conductive element comprises a coil and said current responsive means comprises an electromagnetic relay having normally closed contacts at least one of which is magnetically coupled to the coil, the coil being adapted on current flowing therethrough to open said contacts to disable the heating wire.

6. A circuit according to any of the preceding claims wherein the heating wire dissi pates power at a given rate, the sensing wire comprises a further heating wire adapted to dissipate power at a different rate, and means are provided for selectively energising said wires whereby the wire not energised forms the sensing wire.

7. A protective circuit for an electrical appliance substantially as hereinebefore described with reference to Fig. 3,4,6 or 7 of the accompanying drawings.

CLAIMS (12 Jan 1982)

8. A circuit according to any of claims 1 to 6 wherein said insulating layer has a fusing temperature corresponding to an overheated condition of the electrical appliance.

9. A circuit according to any of claims 1 to 6 wherein said insulating layer has a negative temperature coefficient of resistance whereby an overheated condition of less severity than that required to fuse the insulating layer causes the layer to conduct leakage current through the auxiliary heater before the insulating layer fuses.